Liquid level detection sensors are used to determine a liquid level in a container, in particular the compartment of a liquid tank truck. A liquid detection sensor may for example be arranged at a top side of the interior of the compartment to detect the presence of liquid at this level. The sensor may be used to prevent overfilling of the compartment by stopping filling of the compartment as soon as the liquid level has reached the sensor.
The liquid detection sensor may for example be an optical liquid detection sensor using a light beam to determine the presence of liquid on a liquid contact surface formed by a prism element of the sensor.
The optical liquid level detection sensor comprises a light source for emitting a light beam, a prism element forming liquid contact surface, and a light detector. The light source is arranged to emit a light beam towards the liquid contact surface, which liquid contact surface reflects or transmits the light beam in dependence of liquid being present on the liquid contact surface. The light detector is arranged to receive the light beam after reflection on the liquid contact surface. When the light beam is transmitted through the liquid contact surface due to the presence of liquid on this contact surface, the light beam will no longer reach the light detector. As a result, the presence of liquid on the liquid contact surface may be determined in dependence of the amount of light received by the light detector.
US 2010/0185334 discloses a liquid overfill detection and control system. The system comprises multiple sensors and a controller. The sensors are connected in series to a central processing unit configured to control filling of the compartments of a liquid tank truck. A diagnostic line is connected in parallel to each of the sensors and the controller. Each of the sensors is placed in a liquid compartment to detect whether the liquid level has reached the respective sensor. When one of the sensors detects the presence of liquid at the liquid contact surface filling of the compartments is stopped.
The diagnostic line connected to each of the sensors provides a diagnostic signal indicative of the number of sensors that generate an output pulse, i.e. the number of sensors of which the liquid contact surface is not immersed in liquid.